1. Field of the Invention
The present invention relates to silicon carbide semiconductor devices and methods for their construction. More specifically, the instant invention relates to silicon carbide semiconductor devices with a layer of silicon dioxide disposed on a silicon carbide substrate that have a region of substantial nitrogen concentration at the silicon dioxide-silicon carbide interface.
2. Description of the Related Art
Presently, the silicon (Si) metal-oxide-semiconductor field effect transistor (MOSFET) is the most commonly manufactured transistor in the world. Silicon carbide (SiC) MOSFETS are currently being developed because of superior material properties that provide high power generation, conditioning, transmission and the ability to operate at higher temperatures.
Like Si, SiC thermally oxidizes to form SiO2. However, the oxidation kinetics and oxide interface of SiC are poorly understood in comparison to Si. The oxidation rate of SiC is lower than Si by more than a factor of 10. Furthermore, the quality of the SiO2/SiC interface is also inferior to the SiO2/Si interface. This problem causes low channel mobilities in SiC MOSFETS due to the density of interface states that exists in the band gap near the conduction band edge. Refinements in cleaning and oxidation techniques have improved the interface quality of SiO2/SiC, but producing reliable devices will require additional improvements.
Oxynitride films on Si have superior device reliability, better dopant diffusion resistance and higher dielectric quality than pure oxide films. Methods for manufacturing oxynitride films on Si include annealing SiO2/Si in N2O or NO, with NO believed to be the species responsible for nitrogen incorporation. Nitrogen is typically incorporated near the SiO2/Si interface at concentrations of about 1015 cm−2, although the actual profile is a complicated function of the gaseous species and the temperature cycle. The atomic process for nitrogen incorporation appears to involve NO interstitial diffusion through the oxide to the SiO2/Si interface where a reaction takes place that forms an oxynitride film.
Initial studies of oxynitride films on 6H-SiC have shown a small improvement in MOS interface quality after NO anneal (reduction of the interface state and fixed charge densities takes place) but inferior MOS interface quality after N2O anneal. These results are surprising considering that nitrogen incorporation at the SiO2/Si interface does not decrease the interface trap density. Secondary ion mass spectrometry (SIMS), reveals that NO anneal incorporates nitrogen near the SiO2/4H-SiC interface.
Thus, there exists a need for a method that reduces interface trap density near the conduction band edge at the SiO2/SiC interface. By increasing channel mobility, such a method would facilitate the manufacture of high quality SiC MOSFET devices.